1. Field of Invention
The present invention relates to techniques for narrowing the visual field or increasing the brightness of a surface light source device, by making use of two prism sheets. The present invention is especially advantageously applied to backlighting of a liquid crystal display which is observed preferentially from a certain direction.
2. Related Art
A well-known optical device taking the form of a sheet and acting to modify the propagation direction characteristic of a primary light beam supplied from one surface and to cause the beam to exit from the other surface as a secondary light beam is generally known as a prism sheet.
Generally, a prism sheet consists of a member in the form of a plate made of an optical material having a surface (or prism face) provided with a number of aligned, repeating V-shaped channels (an array of convex and convex portions). Since this device has a function of modifying directions of light flux having a cross-sectional area corresponding to the plasm face, the device is employed, for example, to adjust the illumination direction light for the backlighting of a liquid crystal display.
FIG. 1 is a perspective view diagrammatically showing a surface light source device using a prism sheet for this purpose. Referring to FIG. 1, a light guide plate 1 of wedge-shaped cross section is made of a light scattering guide. For example, the light scattering guide is fabricated by preparing a matrix of polymethylmethacrylate (PMMA) and uniformly dispersing a substance of a different index of refraction in the matrix.
The thick end surface of the wedge-shaped light guide plate 1 forms an incident surface 2. A light source device (fluorescent lamp) L is disposed near the incident surface 2. A reflector 3 consisting either of silver foil of positive reflectivity or of a white sheet of diffuse reflectivity is disposed along one surface (rear surface 6) of the light guide plate 1. The other surface 5 of the light guide plate 1 forms an exiting surface 5 for taking out light flux supplied from the light source L.
A prism sheet 4 is disposed outside this exiting surface 5. One surface of the prism sheet 4 has a V-shaped prism face 4a, 4b. The other surface is a flat surface 4e. If a well-known liquid crystal panel (liquid crystal display device) is disposed further outside the prism sheet 4, a liquid crystal display is given.
In this surface light source device, as the thickness of the light guide plate 1 falls off with distance from the incident surface 2, the surface light source device shows excellent efficiency of utilization of light and excellent brightness uniformity because of oblique surface multiple reflection effect occurring inside the light guide plate 1. This effect based on the shape of the light scattering guide is described in detail in Japanese Patent Laid-Open No. 198956/1995.
Light introduced into the light guide plate 1 from the light source device L is directed toward the thin-walled end surface 7 while being scattered and reflected inside the light guide plate 1. During this process, the light gradually exits from the exiting surface 5. As described later, if the diameters (generally, the correlation distance regarding structures of nonuniform indices of refraction) of particles having a different index of refraction and dispersed in the light guide plate 1 are not very small, light exiting from the exiting surface 5 has a clear direction of preferential propagation. In other words, a collimated light beam is taken out from the exiting surface 5. This nature is hereinafter referred to as the "emitting directivity" or "emission of directional light".
As discussed in detail later, this direction of preferential propagation (the direction of the main axis of the collimated light beam) is usually upwardly spaced about 25-30.degree. from the exiting surface as viewed from the incident surface 2. Taking account of this, the prior art function of modifying the direction of propagation of the prism sheet 4 is described by referring to FIGS. 2 and 3.
FIG. 2 is a diagram associated with the arrangement of FIG. 1, illustrating the behavior of light the cross section taken along a direction vertical to the lamp L. "Direction vertical to the lamp L" means "direction vertical to the long axis of the lamp L", i.e., direction vertical to the direction to which the incident surface 2 extends". This may hereinafter be simply referred to as "lamp-vertical direction". Similarly, "direction parallel to the direction of the long axis of the lamp L", i.e., direction parallel to the direction to which the incident surface 2 extends, may be simply referred to as the "lamp-parallel direction".
The prism sheet 4 shown in FIG. 2 faces the exiting surface 5 of the light guide plate 1 with its prism face directed inward. Preferably, the prismatic vertical angle .phi.3 made in the prism face is about 60.degree.. Prism sheets satisfying this condition and having a prismatic vertical angle .phi.3 of 64.degree. are often used.
The refractive indices of materials of the matrix of the light guide plate 1 are generally about 1.4 to 1.6. Where this is taken into consideration, if light is directed to the light guide plate 1 from the direction indicated by the arrow L', the direction of preferential propagation of light flux exiting from the incident surface 5 forms an angle .phi.2=about 60.degree. with respect to a normal to the exiting surface 5. Where a PMMA matrix having an index of refraction of 1.492 is used, the incident angle to the exiting surface 5 to give 100 2=about 60.degree. is .phi.1=about 35.degree. according to the Snell's law. A light beam corresponding to this direction of preferential propagation will hereinafter be referred to as a representative light beam, which is indicated by B1 herein.
The representative light beam B1 exiting from the exiting surface 5 travels straight through an air layer AR which can be regarded as having an index of refraction n.sub.0 =1.0. Then, the light enters the prism face 4a of the prism sheet 4 at an angle (.phi.3=about 60.degree.) close to a right angle. This light beam enters the opposite prism face 4b at a smaller percentage.
Then, the representative light beam B1 travels substantially straight through the prism sheet 4 up to the opposite prism face 4b and is reflected regularly. The light beam then enters the flat surface 4e of the prism sheet 4 at an angle close to a right angle and then goes out of the prism sheet 4. By this process, the direction of preferential propagation of the light beam exiting from the exiting surface 5 is changed to a direction substantially vertical to the exiting surface 5. The whole light rays are collected into a light beam traveling substantially in the perpendicular direction. As a result, the angular range in which the emitting surface is observed to be luminous is restricted. This action is herein referred to as narrowing of the visual field. The visual field will be quantitatively defined later.
The modified direction of preferential propagation is not always vertical to the exiting surface 5. Rather, the direction can be adjusted in a considerable range of angles by selection of the vertical angle .phi.3 of the prism sheet 4, selection of the material (index of refraction) of the prism sheet 4, selection of the material (index of refraction) of the light guide plate 1, and so forth.
If the prism sheet 4 is so positioned that its prism face is directed outward, the preferential propagation direction is modified by similar prismatic action. In this case, however, the range of preferred prismatic vertical angles is wider than the range obtained where the prism face is directed inward. FIG. 3, which takes the same form as FIG. 2, illustrates this. The vertical angle .phi.4 of the prism made at the prism face is approximately 70.degree..
It is assumed that the direction of incident light lies in the direction indicated by the arrow L'. In the same way as in the case of FIG. 2, a representative light beam B2 corresponding to the preferential propagation direction enters the exiting surface 5 at an angle .phi.4=about 35.degree.. Most of the incident light exits and enters the air layer AR whose index of refraction n.sub.0 =1.0. At this time, the exit angle .phi.2 is approximately 60.degree..
The representative light beam B2 travels straight through the air layer AR and then enters the flat surface 4e of the prism sheet 4 obliquely. The light follows the illustrated refracted path. Finally, the light goes out of the surface 4c of the prism sheet 4 at an angle almost normal to the exiting surface 5. The proportion of the light exiting from the surface 4d is smaller.
Since the path of the light after entering the flat surface 4e of the prism sheet 4 is varied by the index of refraction n.sub.2 of the prism sheet 4 and by the prismatic vertical angle .phi.4, the preferential propagation direction can be adjusted by selecting these parameters. Because the whole light rays are collected substantially into the vertical direction, the visual field is narrowed in the same way as in the case of FIG. 2.
The action of modification of the propagation direction and the action of the narrowing of the visual field of the prism sheet disposed alone function effectively mainly in the plane vertical to the lamp. It is known that the propagation direction is modified less effectively in a plane parallel to the lamp L.
FIGS. 6 and 7 are graphs of data obtained by actual measurements, demonstrating the above-described phenomenon. The conditions under which the actual measurements were made are shown in FIGS. 4 and 5. The fundamental portions are commonly applied to various measurements for examples described later.
Referring first to FIG. 4, the same arrangement as that shown in FIG. 1 is shown. A light guide plate 1 of wedge-shaped cross section comprises a matrix of polymethylmethacrylate (PMMA) having an index of refraction of 1.492 in which 0.08 wt % silicone resin material is uniformly dispersed as a material having a different index of refraction. The particle diameter of the silicone resin material is 2.0 .mu.m. The index of refraction of the resin material is 1.4345. The dimensions are given in the figure.
The end surface which is thicker than the thin-walled end portion 7 of the light guide plate 1 forms the incident surface 2. A fluorescent lamp L having a diameter of 3 mm and taking the form of a long tube is spaced 1.0 mm from the incident surface. A reflective sheet R consisting of silver foil is mounted behind the fluorescent lamp L to prevent scattering of the light. A reflector 3 disposed along the rear surface 6 of the light guide plate 1 is made of a silver foil. A quite thin air layer having a thickness given by .delta.1 exists between the rear surface 6 and the reflector 3.
At the above-described particle diameter of the particles having the different index of refraction, the light guide plate 1 causes directional light to exit from it. A collimated light beam having a preferential propagation direction indicated by 5e exits from the incident surface 5. Measurements were made under the following conditions. Only the first prism sheet PS1 was located outside the exiting surface 5. Alternatively, the first and second prism sheets PS1 and PS2, respectively, were made to overlap each other with the thin air layer AR of the thickness .delta.2 therebetween. Indicated by LP is a liquid crystal display panel positioned when a liquid crystal display is to be constructed. The liquid crystal display panel LP was not positioned during measurements.
Only one prism sheet was positioned for the measurements giving the results shown in FIGS. 6 and 7. The arrangement of each prism sheet used for each measurement will be separately described.
A luminance meter is represented by M (LS110 manufactured by Minolta Co., Ltd.; visual angle of 1/3.degree. closeup lens mounted). The center point P on the outer surface (bright surface) a of the prism sheet PS1 or PS2 was constantly viewed at a distance of 203 mm. The direction of the line of sight b was changed around the central point P. Under these conditions, the luminance meter M was used. Let .phi. be the angle of the line of sight b in the cross section that is vertical to the fluorescent lamp L (general expression of angle .phi.2 used in FIG. 2).
FIG. 5 is a view in which the angle of the line of sight b at the point P on the luminance meter M is generalized to three dimensions. The definition of the angle .phi. is also illustrated. As shown, we now consider a plane c on which the line of sight b which views the central point P is located. The plane c is parallel to the lamp L. The angle that this plane c makes with respect to the normal d to the bright surface a is the above-described angle .phi..
Let a straight line e pass through the central point P on the plane c, the straight line e being vertical to a direction parallel to the lamp. Let .theta. be the angle made between the straight line e and the line of sight b. Let .beta. be the angle that the line of sight makes to the normal line f starting from the central point P. Let .zeta. be the angle that the projection h of the line of sight b onto the bright surface a makes to a direction vertical to the lamp. Since these various measurements were made under conditions in which the direction of line of sight b can be expressed by using only the angles .phi. and .theta., neither the angle .beta. nor the angle .zeta. is cited.
The nomenclatures of the arrangements and postures of the prism sheets PS1 and PS2 are defined as follows.
(1) When the prism surface provided with V-shaped channels is directed toward the light scattering guide as shown in FIGS. 1 and 2, the V-shaped channels are referred to as facing inward. On the other hand, when the prism face provided with the V-shaped channels faces away from the light scattering guide, the channels are referred to as facing outward. PA1 (1) PS1: prismatic vertical angle .psi.=64.degree.; the channels face inward, and the direction of alignment is parallel to the lamp. PA1 PS2: not used PA1 (2) FIG. 6; Measurements were made under the condition .phi.=0.degree.. At this time, the angle .theta. was scanned in the range of from -80.degree. to +80.degree.. This scanned angle is plotted on the horizontal axis. PA1 (3) The luminance value is plotted on the vertical axis in units of 1000 nt, where nt=cd/m.sup.2. The plotted luminance value has been subjected to cosine correction to remove the factor (proportional to the inverse of the cosine of the inclination angle) contained in the output from the luminance meter M when the line of sight b is inclined with respect to the bright surface a. In the case of FIG. 6, .theta. is the inclination angle. In the case of FIG. 7, .phi. is the inclination angle. With respect to all other graphs, values subjected to cos corrections are employed as plotted luminance values. PA1 (4) Explanation; It can be seen from both graphs that the measured peaks lie in the direction .theta.=.phi.=0.degree., i.e., the front of the surface light source device. For both graphs, the whole shape assumes a hill-shaped profile which is substantially symmetrical with respect to the peak and has feet. PA1 (1) It is possible to select either the inward arrangement of the channels or outward arrangement of the channels for each of the prism sheets PS1 and PS2 (see FIG. 4). The latter arrangement is necessary for the present invention. PA1 (2) Orientations of the prism sheets PS1 and PS2. That is, it is possible to select either the lamp-parallel orientation or lamp-vertical orientation. In the present invention, where the prism sheet PS1 is placed parallel to the lamp, the prism sheet PS2 is placed vertical to the lamp.
(2) When the prism sheets are so arranged that the direction of alignment of the V-shaped channels in the prism face is parallel to the fluorescent lamp L (incident surface 2) as shown in FIGS. 1 and 2, the direction of alignment is referred to as being parallel to the lamp or simply as being lamp-parallel. On the other hand, when the prism sheets are so arranged that the direction of alignment of the V-shaped channels formed in the prism face is vertical to the fluorescent lamp L (incident surface 2), the direction of alignment is referred to as being vertical to the lamp or simply as being lamp-vertical.
It is also assumed that the vertical angles of the prism sheets PS1 and PS2 are given by .psi.. This notation is a generalization of .phi.3 shown in FIG. 2 or .phi.4 shown in FIG. 3. Description of data obtained by actual measurements is hereinafter given clause by clause. In the following description, a word "visual angle" which may also be known as "viewing angle" is used as an index representing the range of angles at which the bright surface is observed to be luminous. The visual angle is defined in a plane parallel to the lamp and also in a plane vertical to the lamp. The value is given by such a notation that the half-value width of the graph obtained by each measurement is located in the center, or 0.degree. (e.g., .+-.30.degree.).
[Graphs of FIGS. 6 and 7]
FIG. 7; Measurements were made under the condition .theta.=0.degree.. At this time, the angle .phi. was scanned in the range of from -80.degree. to +80.degree.. This scanned angle was plotted on the horizontal axis.
However, it can be seen from the spread of both graphs that the visual field in a plane vertical to the lamp is very narrow but the visual field in a plane parallel to the lamp is considerably broad. For detailed values obtained from actual measurements, refer to Table 3 given later. In particular, in the present example using one prism sheet, narrowing of the visual field is accomplished in the plane vertical to the lamp but was not in the plane parallel to the lamp.
Although the data obtained from actual measurements is omitted for the arrangement where the prism face of the prism sheet 4 faces outward, narrowing of the visual field is also accomplished in the plane vertical to the lamp but not in the plane parallel to the lamp.
Many liquid crystal displays are observed preferentially from the front direction with respect to both vertical and horizontal directions. Back lighting applied to these liquid crystal displays is, of course, required to accomplish narrowing of the visual field in both vertical and horizontal directions, i.e., in the plane vertical to the lamp and also in the plane parallel to the lamp. Also, even if the preferentially observed direction is rather spread and conspicuous narrowing of the visual field is not required, the visual field is preferably restricted to some extent in the plane vertical to the lamp and in the plane parallel to the lamp, for the following reason.
If illuminating light propagates in the direction to which observation is hardly expected, i.e., in a direction in a greatly deviating from the front direction, e.g., in a direction spaced more than 30.degree. from the front direction, then increase in the brightness of the surface light source device is hindered. This deteriorates the display quality of a liquid crystal display incorporating the light source device.
However, in the above-described well-known usage of prism sheets, it is impossible to satisfy the requirement, i.e., narrowing of the visual field both in the plane vertical to the lamp and in the plane parallel to the lamp.
Furthermore, even if intensive narrowing of the visual field is not required, it is difficult for the prior art techniques to accomplish considerable narrowing of the visual field in the plane vertical to the lamp and in the plane parallel to the lamp, thereby increasing the brightness of the surface light source device. The former (intensive narrowing of the visual field) is hereinafter referred to as "narrowing of the visual field". The latter (increase of the brightness caused by a considerable narrowing of the visual field) is hereinafter referred to as "increase of the brightness".